1. Field of the Invention
This invention relates to an impact sensing apparatus; and more particularly to an apparatus used as a sensor for starting the operation of a safety system such as an airbag or seat-belt tensioner to protect against impact.
2. Description of the Related Art
A conventional impact sensing apparatus used as a sensor for starting the operation of a safety system is disclosed in the U.S. Pat. No. 5,142,922. The impact sensing apparatus disclosed in the prior art comprises a housing and a pair of circular-shaped weights which are swingably supported in the housing. A firing lever, which engages the weights, is rotatably supported in the housing, and a spring member is disposed between the housing and the firing lever for rotating the firing lever by its elastic force. The spring member holds the weights against swing movement at the initial position where the outside of the weights are in contact with the housing. In accordance with the structure of the above impact sensing apparatus, the firing lever disengages the weights by the swing movement of the weight when an impact, exceeding a predetermined value, is applied to the impact sensing apparatus. Thus, the firing lever is rotated by the elastic force of the spring member, and the safety system is operated.
A related art of an impact sensing apparatus used as a sensor for starting the operation of a safety system is disclosed in the U.S. Pat. No. 5,193,407 issued on Mar. 16, 1993. The impact sensing apparatus disclosed in the related art comprises a housing on which a plate is fixed. A circular-shaped weight is rotatably and slidably supported in the housing. A firing lever, which engages the weight, is rotatably supported in the housing, and a spring member is disposed between the housing and the firing lever for rotating the firing lever by its elastic force. The spring member holds the weight against its rotational and sliding movement from its initial position where a stopper pin, mounted on the weight, contacts the plate. In accordance with the structure of the above impact sensing apparatus, the firing lever is disengaged from the weight by the rotational and sliding movement of the weight when an impact, exceeding a predetermined value, is applied to the impact sensing apparatus. This permits the firing lever to be rotated by the elastic force of the spring member, and the safety system is operated.
However, in accordance with the first impact sensing apparatus mentioned above, because the weights are held by the spring member against swing movement at their initial position where the outside of the circular-shaped weights are in contact with the housing, each of the portions of the weights in contact with the housing are adjacent to each of the swingable center points of the weights; and the elastic force of the spring member is applied to each of the contact portions in a direction parallel to one wall of the housing. Therefore, minute deformation of the housing generated by the contacts of the weights with the housing, increase the frictional force between the weights and the housing. Consequently, because the weights adhere to the housing, the impact sensing apparatus cannot operate smoothly, even though an impact exceeding a predetermined value, is applied to the impact sensing apparatus.
In accordance with the second impact sensing apparatus mentioned above, the initial position of the weight varies either because of the dislocation of the stopper pin at a predetermined position, or the deformation of the stopper pin by stress occurring at the contact portion of the stopper pin and the plate. Therefore, the firing lever of the impact sensing apparatus sometimes does not completely engage the weight.
Additional art related to the impact sensing apparatus used as a sensor for starting the operation of a safety system is disclosed in the Japanese Patent Laid Open No. 60(1985)-248455. The impact sensing apparatus disclosed comprises a housing, a sensing mechanism disposed in the housing for starting the operation of the safety system upon an impact exceeding a predetermined value, and a safety mechanism is disposed in the housing for preventing the sensing mechanism from inadvertently operating before the impact sensing apparatus is mounted on an assembly, such as a steering mechanism of a vehicle.
In accordance with the above apparatus, when an impact exceeding a predetermined value is applied to the impact sensing apparatus, weights of the sensing mechanism are moved by the impact. Firing levers of the sensing mechanism are rotated by the weights, which force one end of each of the firing levers. Therefore, firing pins of the sensing mechanism disengage cam portions, each of which are disposed on the firing levers. The firing pins are projected out of the housing by the elastic force of spring members of the sensing mechanism for igniting a pyrotechnic element such as a primer. Consequently, the safety system is operated.
The structure and the operation of the safety mechanism will be described hereinafter. Before the impact sensing apparatus is mounted on the assembly, such as a vehicle, lock arms of the safety mechanism, forced by spring members, engage the weights and prevent the weights from moving. When the impact sensing apparatus is mounted on the assembly, a lock pin disposed on the assembly is inserted into the safety mechanism, and each of the lock arms is turned because of the engagement between each of the lock arms and the lock pin. Therefore each of the weights are disengaged from the lock arms, and the weights are permitted to move. That is to say, before the impact sensing apparatus is mounted on the assembly, the operation of the sensing mechanism is restrained by restricting the movement of the weights. When the impact sensing apparatus is mounted on the assembly, the operation of the sensing mechanism is permitted because the weights are free to move. Therefore, the inadvertent operation of the impact sensing apparatus is prevented before the impact sensing apparatus is mounted on the assembly.
However, in accordance with the above structure, because each of the lock arms is turned by the lock pin, which engages the lock arms, the impact sensing apparatus requires the precise location of the lock arms relative to the lock pin so that the lock arms may completely engage and disengage the weights. Therefore, the impact sensing apparatus is inconvenient, which increases the cost.